With increased volumes of hard disks in recent years, it is necessary to improve precision of hard disk drives. Thus, balance-adjusting weights (hereinafter referred to as “balancers”) are frequently used around disks of hard disk drives.
Since materials having higher specific gravities and good cleanliness are required as crosslinked rubber products to be used for balancers, crosslinked fluororubber products have been used conventionally.
Typical crosslinked fluororubber products, however, have specific gravities of about 1.8 to 2.1, so that several balancers are sometimes required to be used to adjust a balance of a disk, thereby problematically increasing a cost due to the several balancers.
Further, when disks are out of balance, it is sometimes impossible to fully balance them using balancers with the specific gravities conventionally used for balancers.
In view of the above, crosslinked rubber products having higher specific gravities while maintaining cleanliness are demanded for balancers. As a technique to increase a specific gravity of a crosslinked fluororubber product, it is known to blend barium sulfate thereinto, as a filler having a higher specific gravity and having less influence on physical properties of the crosslinked fluororubber product.
Blending of large amounts barium sulfate to achieve increased specific gravities of 2.2 or more, however, results in occurrence of influence on physical properties of the balancers, and particularly results in considerably decreased breaking elongations of crosslinked rubber products, such that molding of products is made difficult depending on the shapes of the products.
Meanwhile, it is a recently growing tendency to decrease amounts of halogens (particularly, chlorine and bromine) in the whole fields of electric and electronic components, so that balancers are also required to have lower halogen content.
Further, rubber surfaces are to be desirably non-tacky to metals so that balancers can be smoothly mounted, and breaking elongations of crosslinked rubber products are to be desirably large so that the crosslinked rubber products can be molded without difficulties even when shapes of the balancers are complicated.
In view of these points, examples of performances or physical properties required for crosslinked rubber products for balancers include: (1) to be clean (that is, to have anti-static properties sufficient to avoid attraction and adhesion of dust), (2) to have a higher specific gravity, (3) to have a better physical property in terms of breaking elongation, (4) to be free of halogen substance (chlorine), and (5) to have a rubber surface which is non-tacky to metal(s).
WO 2004/094479 discloses that a lower friction can be attained by using, for a polyol-crosslinkable fluororubber, bisphenol AF as a crosslinking agent and triphenylbenzylphosphonium chloride as a crosslinking accelerator.
However, although this technique is not so problematic in the points (1) to be clean, and (5) to be non-tacky to metal(s), this technique entails chlorine in the crosslinking accelerator, thereby problematically failing to fully meet the requirements.
WO 2007/058038 discloses that a lower friction can be attained by using, for a polyol-crosslinkable fluororubber, bisphenol AF as a crosslinking agent, and 5-benzyl-1,5-diazabicyclo[4.3.0]-5-nonenium tetrafluoroborate as a crosslinking accelerator.
However, although this technique is not so problematic in the points (1) to be clean, (4) to be free of halogen substance, and (5) to be non-tacky, blending of a large amount of filler having a higher specific gravity for an increased specific gravity results in deteriorated physical properties correspondingly to the blending amount, so that molding of products is problematically made difficult depending on shapes of the products.
Although Japanese Patent Publication JP2007-169511 uses, for a peroxide-crosslinkable fluororubber, 2,5-dimethylhexane-2,5-dihydroperoxide as a crosslinking agent and triallyl isocyanurate as a crosslinking aid, and uses triphenylbenzylphosphonium chloride for a lower friction in a manner that this technique is not so problematic in the points (1) to be clean and (5) to be non-tacky, this technique contains chlorine in the component for the lower friction, thereby failing to fully meet the requirements. Japanese Patent Publication JP2007-137977 discloses a fluororubber composition comprising a fluororubber crosslinkable with a polyol and uses an ammonium salt as a crosslinking aid and a polyol as a crosslinking agent, which is further blended with an electrically conductive additive.